1. Field of the Invention
The present invention relates to a diversity system and a diversity method for implementing space diversity using a carrier unit for OFDM (Orthogonal Frequency Division Multiplex) demodulation.
2. Discussion of the Related Art
In recent years, OFDM has been used as one mode of demodulation as a standard for digital television (Integrated Services Digital Broadcasting-T: ISDB-T), wireless LAN (IEEE 802.11a) or the like. The OFDM is a mode of modulating a great number of carriers that are orthogonal to each other with digital data to multiplex and transmit these modulated waves. In this OFDM mode, the transmitted OFDM signal is provided with a guard period that a portion of the transmitted waveform is copied. By utilizing this feature, the OFDM mode can prevent deterioration in the quality of signal received through multiple paths having a delay time not more than the guard period.
Ground wave digital TV broadcasting such as ISDT-T is also used for mobile reception such as a television receiver mounted in an automobile. In this case, frequency selective phasing occurs for mobile reception and the reception characteristics deteriorate due to Doppler shift or the like. In the case of analog broadcasting, a plurality of antennas are used for receiving signals at the time of mobile reception. Thus, a signal corresponding to a antenna series that enables better reception is selected in accordance with reception levels. This mode is referred to as selection diversity.
In contrast to this, selection or weighted combination can be carried out in digital broadcasting for each carrier of the OFDM signal, in addition to selection of the received RF signal or baseband signal that is carried out in the above described mode. This mode is referred to as selection and combination diversity.
One example of weighted combination will be described. A symbol received from a first antenna series in a symbol period at time t1 is denoted as S1 and a symbol received from a second antenna series is denoted as T1. When it is assumed that these symbols are modulated according to multiple value PSK, for example, the symbols T1 and S1 have the same phase angle and amplitude at the transmission end. When the imaginary number unit is denoted as j, the symbol S1 obtained at the reception end becomes a two dimensional vector S1=sx+jsy, and the symbol T1 obtained at the reception end becomes a two dimensional vector T1=tx+jty. When the reliability of the received symbol S1 is denoted as RS1 and the reliability of the received symbol T1 is denoted as RT1, the received symbol U1 on which weighted combination is carried out is represented as:U1=[RS1*(sx+jsy)+RT1*(tx+jty)]/(RS1+RT1).
Selection and combination diversity is to compare the vector U1 with a known vector that is the closest to the vector U1 and to estimate the transmitted signal. It is noted that this operation is carried out on data obtained by implementing an equalization process for the respective antenna series.
The above-described selection and combination diversity mode for selecting or weighted combining for each carrier of the OFDM signal utilizes a mode of using one antenna series as a master and the other antenna series as a slave among a plurality of antenna series. Thus, preservation of temporal synchronicity of carriers among antenna series is a prerequisite condition of the above-described selection and combination diversity mode.
In some cases, however, the reception condition of the master antenna series significantly deteriorates during reception, so that OFDM synchronicity is lost. In this case, a synchronous signal from the master to the slave is lost and reception becomes impossible even in the case the reception conditions of slave antennas are good. In contrast to this, the above-described problem does not arise in a mode of independently detecting the synchronous signal of the OFDM symbol by two antenna series.
Symbol synchronicity according to OFDM is generally implemented through correlation of the guard period with a portion of the OFDM waveform. The correlation waveform according to OFDM is independent of each antenna series; therefore, subtle differences in the correlation waveform of each series greatly affects the shift in the timing of the symbol synchronous signal. In particular, the timing of the symbol synchronous signal greatly varies among the antenna series under a mobile reception environment and there has been arisen a problem in that diversity use is difficult.